During recent decades there has been a surge of activity in the use of both low temperature liquids and pressurized gases to perform a variety of operations at sub-freezing temperatures. The activity of this field has been particularly pronounced in devices designed for cryosurgery purposes on both human and animal life. Designers and users of such equipment are confronted with numerous problems for which fully satisfactory solutions have not been achieved. Devices using high pressure gases have the decided advantage that the gas can be stored indefinitely at room temperature in a container of adequate strength. Such storage chambers are commonly pressurized to 800 lbs. or 1,000 lbs. or more and this presents a problem and hazards if such high pressure gas is released into an expansion chamber designed for contact with living tissue to be treated and desirably formed of thin-walled material having excellent heat conductivity. Another major problem involves the low heat conductivity of frozen tissue making it difficult to freeze to a depth of more than a few millimeters during a single freezing cycle. After thawing, frozen tissue may be removed to facilitate another freezing cycle but this necessitates appropriate provision for interrupting the operation of the cryogenic device and providing suitable means for thawing and removing the frozen tissue.
Another particularly serious problem involves the control of the high pressure gas which typically has necessitated the application of objectionably high manual operating forces to manipulate the controls, the need for applying these control forces over a prolonged period, the highly objectionable noise heretofore inherent in the operation of these controls along with the accompanying shock and vibration associated with the opening and closing of high pressure valves. These problems have seriously interfered with the use of cryosurgery devices in performing delicate operations on the brain, eye, ear and the like.
Typical patents disclosing structures subject to the numerous short-comings of the prior art include U.S. Pat. Nos. Hart 3,495,419; Amoils 3,502,081; Crump et al. 3,512,531; Reynolds et al. 3,548,829; Wallach 3,696,813 and 4,018,227; Stumpf 3,807,403; Ritson 3,913,581. Hart has a simplistic design utilizing pressurized gas and operable only in a freezing mode. Amoils discloses a much more sophisticated device employing an electric heater for defrosting and a thermocouple in the expansion chamber to measure temperature. Crump et al., Reynolds et al., the eariler Wallach patent and Stumpf each employ an exhaust valve effective to provide momentary or short duration defrosting capability at the end of a freezing cycle. In both Crump et al,. and Reynolds et al., the exhaust valve is normally closed with the result that the expansion chamber is pressurized at the pressure of the supply source which is hazardous to both the equipment and personnel for obvious reasons. Both Ritson and the later Wallach patent utilize normally open exhaust valves operable to provide short duration defrosting and including provision for purging the gas supply orifice opening into the expansion chamber.